This invention relates to aqua-ammonia absorption cooling and/or heating systems utilizing ammonia refrigerant and aqueous absorbents. Improvements in the efficiencies of such systems includes the use of generator/absorber heat exchange cycles utilizing rich and weak absorption working fluids and/or by separate heat exchange loops referred to as GAX cycles. Descriptions of such systems are found in U.S. Pat. Nos. 4,311,019, 5,024,063, 5,271,235, 5,367,884, U.S. Pat. No. Re. 36,684 and R. J. Modahl and F. C. Hayes, xe2x80x9cEvaluation of Commercial Advanced Absorption Heat Pump Bread Board,xe2x80x9d The Trane Company, pp. 117-125, 1988. Additional improvements are described in co-pending U.S. patent application Ser. Nos. 09/479,277, filed Jan. 5, 2000; 09/632,037, filed Aug. 3, 2000; 09/632,054 filed Aug. 3, 2000; and 09/709,875, filed Nov. 10, 2000. The description of the aforesaid patents and applications are incorporated herein by reference.
The present invention is directed to further improvement in efficiency of aqua-ammonia absorption systems. The circulation rate of the weak absorption fluid through an aqua-ammonia absorption system is an important factor in proper operation. The weak solution from the bottom end of the generator, also referred to herein as xe2x80x9cweak liquorxe2x80x9d, has the lowest concentration of ammonia in the absorption cycle, typically between about 2% and about 20% ammonia, depending on operating mode (heating or cooling), operating conditions, and system design parameters. Excess ammonia in weak liquor decreases the amount of refrigerant generated for a given circulation of solution through the absorption cycle. Thus, the circulation rate must be increased to generate the required amount of refrigerant. Increased solution circulation rate increases the required heat input to the generator, and decreases system efficiency. This efficiency degradation is especially pronounced for GAX systems where circulation rate must be sufficiently high to absorb all refrigerant passing through the condenser and evaporator, but low enough for weak liquor to be adequately depleted in ammonia, which is necessary for the solution to absorb ammonia refrigerant. Moreover, in the case of strong-liquor GAX, ammonia depletion is important to allow the GAX section of the absorber to operate at temperatures sufficiently high for heat transfer to the strong liquor, i.e., absorption fluid solution having relatively high ammonia concentration. Weak liquor with excessive ammonia concentration and low bubble-point temperature reduces the amount of heat that can be recovered in the GAX absorber, thereby reducing system efficiency. Active control of weak liquor flow is needed for optimizing efficiency of the system by maintaining ammonia concentration in the weak liquor within a desired range when temperature conditions change or in systems where firing rate is changeable. As used herein, active weak liquor flow control is intended to mean control and adjustment of weak liquor flow in response to changes in the system or operating conditions in contrast to use only of a fixed restriction such as a fixed orifice or capillary tube. Such weak liquor flow control may also be used advantageously with systems having variable speed or multiple speed burners or firing rates. The terms xe2x80x9cburner speedxe2x80x9d and xe2x80x9cfiring ratesxe2x80x9d are used interchangeably herein.
As described herein, the flow of weak liquor from the bottom of the generator to the absorber is actively controlled using one or more valves, operated or adjusted in response to a condition or property of the weak liquor, for example, temperature, pressure, concentration and/or, in cooperation with operating conditions, temperatures and/or other sensible parameters.
In one embodiment, a valve and a temperature-sensing device in thermal contact with weak liquor are used to actively control the flow of weak liquor from the bottom of the generator to the absorber assembly. A preferred valve is capable of a continuous adjustment of the weak liquor flow in response to sensed temperature. In another embodiment, weak liquor flow is actively controlled in discrete steps rather than by continuous adjustment.
In another embodiment, weak liquor flow control is used in a system also using active control of refrigerant flow to the evaporator for controlling vapor superheat.